1. Field of the Invention
The present invention relates to a semiconductor device including an oxide semiconductor and a method of manufacturing the semiconductor device.
In this specification, a “semiconductor device” generally refers to a device that can function by utilizing semiconductor characteristics; an electrooptic device, a semiconductor circuit, and an electronic device are all included in semiconductor devices.
2. Description of the Related Art
A technique for forming a thin film transistor (TFT) by using a semiconductor thin film formed over a substrate having an insulating surface has been developed. Thin film transistors are applied to a wide range of electronic devices such as ICs and electrooptic devices, and are used as switching elements in image display devices (liquid crystal display devices and EL display devices) in particular.
An image display device (liquid crystal display device or EL display device) including thin film transistors as switching elements is called active matrix display device. An active matrix display device has the following advantage: as driver circuits for transmitting signals to a pixel portion, circuits such as a shift register circuit, a latch circuit, and a buffer circuit can be formed using TFTs on the same insulator.
However, an active matrix display device has also a disadvantage that manufacturing cost is increased when a manufacturing process of TFTs is complex. Further, since a plurality of TFTs are formed concurrently, complexity of a manufacturing process makes it difficult to ensure the yield. In particular, malfunction in a driver circuit sometimes causes a linear defect in which pixels in one column do not operate.
Disclosed in Patent Document 1 is a technique for forming a pixel portion and a driver circuit with the use of only n-channel transistors.
In addition to display devices, other various semiconductor integrated circuits such as LSIs can be manufactured by using an inverter circuit (NOT circuit), a buffer circuit, and the like in combination. In general, an inverter circuit, a buffer circuit, and the like are formed using a CMOS circuit in which an n-channel TFT and a p-channel TFT are used in combination.